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Put the -1 on the greens along the chroma axis, not across it

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The Malvar "R at green in R row" kernel weights the two greens two
sites away along the row at -1 and the pair up and down the column at
+1/2. The shader had the two swapped, in the comment as well as the
code, so the transcription checked against itself. Both sum to zero
and reconstruct a flat patch exactly, which is all the tests fed it.

On an edge the correction at green sites is half strength and the
false colour doubles: 0.375 against 0.19 on a grey step, and a
blue/yellow zipper around every clipped highlight at 1:1. The other
three kernels and the CFA tables were right.

A grey vertical step now runs through the pass; the transposed kernel
fails it at 0.375.
This commit is contained in:
dtourolle
2026-09-19 22:05:39 +02:00
parent b83f192847
commit c6cfb2a02a
2 changed files with 57 additions and 4 deletions
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core/dr-gpu/src/shaders/demosaic.wgsl
+10 -4
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@@ -160,12 +160,18 @@ fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
// Green is measured. Red and blue are interpolated from their own
// axis, with a correction from the green Laplacian.
//
// Malvar "G at R/B locations" kernels, transposed per axis:
// chroma along the row: (5c + 4(w1+e1) - (nw+ne+sw+se) - (n2+s2) + 0.5(w2+e2)) / 8
// Malvar "R at green in R row" kernel, and its transpose:
// chroma along the row: (5c + 4(w1+e1) - (nw+ne+sw+se) - (w2+e2) + 0.5(n2+s2)) / 8
//
// The -1 goes on the two greens *along* the chroma axis and the +0.5
// on the pair across it. Transposed, both kernels still sum to zero
// and reconstruct a flat patch exactly, but on an edge the correction
// at green sites is half strength and the false colour doubles: a
// blue/yellow zipper around every clipped highlight.
let along_row =
(5.0 * c + 4.0 * (w1 + e1) - diag1 - vert2 + 0.5 * horiz2) * 0.125;
(5.0 * c + 4.0 * (w1 + e1) - diag1 - horiz2 + 0.5 * vert2) * 0.125;
let along_col =
(5.0 * c + 4.0 * (n1 + s1) - diag1 - horiz2 + 0.5 * vert2) * 0.125;
(5.0 * c + 4.0 * (n1 + s1) - diag1 - vert2 + 0.5 * horiz2) * 0.125;
let red_horizontal = red_is_horizontal(gid.x, gid.y);
let r = select(along_col, along_row, red_horizontal);